1. Field of the Invention
The present invention relates to a radio apparatus, and more particularly to a portable, compact digital mobile radio apparatus.
Car telephones and other mobile-portable telephones perform communication using allocated radio frequencies in the 800-MHz and 1.5-GHz bands. Going digital in these systems reduces the cost of base stations, and using a time-division multiple access (TDMA) technique, the allocated radio frequencies can be used efficiently. For such mobile radio communication systems, the need has been increasing to reduce the size and weight of digital mobile radio apparatus.
2. Description of the Related Art
In conventional mobile units for digital mobile radio communication, both the receiver and transmitter are connected to a receive/transmit common antenna via a transmitting band-pass filter and a receiving band-pass filter respectively. The transmitting band-pass filter permits signals only within the transmitting frequency band to pass through and thus keeps received waves from entering the transmitter, while the receiving band-pass filter permits signals only within the receiving frequency band to pass through and thus keeps transmitted waves from entering the receiver. Furthermore, in the transmitter unit, an isolator for limiting the passage of signals in only one direction is inserted between the transmitter and the transmitting band-pass filter, to prevent the entrance of electric waves radiated from other mobile units in the same frequency band but at a different operating frequency. The transmitting band-pass filter and receiving band-pass filter are each constructed from a dielectric filter or a surface acoustic wave filter.
Electronic circuitry for mobile radio communication equipment has been reduced in size and weight by using semiconductor integrated circuits. However, in prior art mobile radio communication apparatus, the transmitting band-pass filter and receiving band-pass filter are constructed from a dielectric filter, a surface acoustic wave filter, or the like, as noted above, but these components are difficult to fabricate as integrated circuits. This has placed a limitation on the reduction of the overall size.